In telecommunications systems for sending information via a communications channel, the information needs to be modulated; in other words, it needs to be adapted and matched to the channel.
Some of the main problems that a receiver of digital communications encounters are equalization, which entails an estimation of the channel, and frequency deviation of the receiver clock with respect to the transmitter. When QAM (square amplitude modulation) coherent modulations are used, where the point of the constellation is determined by the amplitude and phase of the signal sent, the demodulator has to be made much more complex in order to tackle the problems stated above. One solution consists of using differential modulations or DAPSK (amplitude and phase differential modulation) where the information is coded in the amplitude and phase increments. This differential coding eliminates the need for estimation of the channel in reception and to a large degree minimizes the effects of frequency deviation between the clocks. But this reduction in the complexity of the receiver when using a totally differential modulation is associated with an increase in the bit error probability for the same SNR (signal to noise ratio) value compared to the performance of QAN modulations. ADPSK modulation (amplitude differential phase-shift keying modulation) is known in the state of the art as it appears in “Comparison and optimization of differentially encoded transmission on fading channels”, L. Lampe and R. Fischer, Proceedings ISPLC'99; “Performance evaluation of non-coherent transmission over power lines”, L. Lampe, R. Fischer and R. Schober, Proceedings ISPLC'00; “Differential encoding strategies for transmission over fading channels”, R. Fischer, L. Lampe and S. Calabró, International Journal on Electronics and Communications; as a mixed modulation between the above two and which represents the intermediate point in terms of performance and complexity of receiver between them both. In other words, it minimizes the problem implied by frequency deviation between the clocks of the transmitter and receiver in a digital communications system and decreases the complexity of estimation of the channel since it is only necessary to estimate the channel in amplitude. So, ADPSK modulation represents the best compromise between performance and complexity of the receiver for a practical implementation.
Moreover, in order to obtain at all times the maximum data transfer rate, the bits per carrier of the modulation need to be adapted to the capacity offered by the channel. In other words, given a target bit error probability, the aim is to use the maximum number of bits per carrier that ensure an error probability equal to or less than that value. Also, in a point to multipoint or multipoint to multipoint multiuser communication, a transmitter can send information to several receivers with different channels between that transmitter and each of the receivers in a single data frame. Therefore, different constellations will be used in the same frame. So, it is necessary to estimate the signal-to-noise rate (SNR) perceived by the receiver in order to choose the number of bits per carrier to use.
Another important factor for achieving that maximum data transfer rate is to minimize the overhead (control information on the system necessary for a correct reception of the data and which is sent along with it). This overhead is more important if transmission strategies are used based on the use of multiple carriers such as OFDM (orthogonal frequency division multiplexing) where the symbol times are much greater and contain a lot more information than in a digital communication in which a single information carrier frequency is used.
ADPSK modulation has two important requirements. The first is that, owing to the fact that part of the information is coded in the phase increments, a symbol previously needs to be sent constituting a phase reference for the receiver. Also, the rest of the information is coded in the value of the amplitude of the received symbol. So, the second requirement implies estimating the value of the amplitude of the response from the channel in order to correct its effect in the receiver. Moreover, real channels display a certain variation in their characteristics with time, which compels the receiver to conduct a monitoring and updating of that initial estimation. In addition, this time variation also requires a continual updating of the estimation of the SNR.
The articles cited above describe the functioning of the ADPSK modulator and demodulator and its performance, without considering the possibility of employing different constellations within a single data frame. This possibility is considered and resolved in the Spanish patent application P-200301120 (unpublished), referring to a “Procedure for phase differential amplitude coherent modulation standardized for multiuser communication”, which permits the sending of a single phase reference at the beginning of the frame and facilitates a practical implementation of the low-complexity modulator.
Moreover, this procedure permits the insertion of data symbols in the frame in such a way that users to which the data from the transmitter is not directed, and who do not know the constellation with which it is modulated, can monitor the channel and follow its variations in both amplitude and SNR.
Therefore, in order to optimize the data transfer in a multiuser communication, it is necessary to make an estimation of the SNR perceived by the receiver. This estimation can only be made when the constellation in which the received data is modulated is known by the receiver. Also, the period during which the SNR is estimated can include symbols modulated with different constellations.
Thus, the problem to solve consists of estimating the SNR in the receiver of a signal with ADPSK modulation during a period comprising reception of a certain number of data symbols with the possibility that they are modulated using different constellations. In the article “A comparison of SNR estimation techniques for the AWGN channel”, D. Pauluzzi and N. Beaulieu, IEEE Transactions on Communication, vol. 48, No 10, October 2000, various techniques are presented for estimating the SNR of a signal with coherent phase modulation (PSK). Also, it is stated how to extend those techniques to a QAM modulation. In both cases, no account is taken of the fact that the constellation can change during the estimation time.
The average power of the transmitted signal can be known if the constellation is normalized in power and the effect of the channel in reception is equalized. Then, in order to estimate the SNR in the receiver, it is merely necessary to estimate the noise power in the received constellation. This estimation of the noise power is easily done by means of averaging the samples of the noise power. So, the problem consists of calculating those samples in the receiver. But another added problem is the differential character of the phase in ADPSK modulation; in this case the constellation received is the constellation formed by the amplitudes and the phase increments that are received. If the value of the samples of the noise power is obtained by means of calculating the modulus squared of the noise vector given by the error in amplitude and by the error in phase increment, without any modification, it is observed that the estimation presents a greater variance in the constellations of odd bits per symbol. This effect is not admissible because the period during which the SNR is estimated can include symbols modulated with different constellations in an optimum multiuser communications system. This communications system also includes transmissions aimed at multiple users (multicast) or at all of them (broadcast), as well as transmission to a single user (unicast).
The procedure of the proposed invention presents a method of estimating the SNR of a signal with ADPSK modulation which equalizes the variances of the estimation in constellations with even and odd bits per symbol, furthermore reducing the variance of that estimation for all cases.